general you right, you can have fun on boards even more than 20 cm width gap too. but unregistered wrote, he wants to use this two boards for slalom racing.
in slalom racing when you swich from iSonic 133 (85 wide) to the falcon 102 (63 wide), because the 133 is over powered and slowing down, in this condition with the 102 you will stop after every jibes at the marks. in this area at the leeward side of the marks are wind holes and difficult to keep the board in planning (around you other 7 hungry guys). so if the board is not enough wide you cannot pump into planning fast.
for slalom racing 22 cm board width different is too big to cover a homogene wind range.